The present invention relates to an instrument for making static attitude (angular orientation) and vertical offset distance measurements (Z-height) of elements of a head suspension assembly (HSA). In particular, the present invention is an instrument that includes an autocollimation system and a point range sensor.
HSAs position a read/write head over the spinning surface of a data storage device (e.g. a magnetic hard disk). HSAs are some of the smallest and most delicate components of a rigid disk drive. An HSA includes a suspension assembly, an elongated spring structure, with a head assembly positioned at a distal end. Suspension assemblies act in a similar fashion to the needle arm in a record player, positioning the head assembly nanometers from the surface of a spinning disk in the disk drive. Typical suspension assemblies measure less than 20 mm long and are 0.03 to 0.1 mm thick. Suspension assemblies generally include an elongated load beam with a gimbal flexure located at a distal end and a base plate or other mounting means located at a proximal end.
The gimbal flexure comprises a head bonding platform suspended by spring arms. The head assembly is mounted to this head bonding platform. The head assembly includes an air bearing slider and a read/write magnetic transducer formed on the slider. The slider is aerodynamically shaped to use the air stream generated by the spinning disk to produce a lift force. During operation of the disk drive, the spring arms provide gimballing motion to maintain the head assembly at a desired orientation with respect to the surface of the disk. The suspension assembly must balance the different lift forces on the outside and the inside air-bearing surfaces of the slider (the outside circumference of a round disk has a faster linear velocity than the inside, and therefore produces more lift), static forces (e.g. weight and pressure applied on the slider by the suspension assembly), and dynamic forces (e.g. momentum). The gimbal flexure and the whole HSA are manufactured within precise tolerances.
In a magnetic disk drive, the density and accuracy of the data stored on the disk depend on the distance and attitude of the head assembly with respect to the surface of the disk. The size of the magnetic field "spot" written and read by the transducer is directly proportional to the square power of the distance between the transducer and the disk. Small changes in distance and/or attitude can cause the head assembly to "crash", that is, to hit the surface of the spinning disk. A crash can destroy both the transducer and the data on the surface of the disk. Tight manufacture tolerances are a factor in determining disk drive reliability.
HSA manufacturers must repeatedly measure and control the Z-height and static attitude of different elements of the HSA at various points during the manufacturing process. The reference point for both the Z-height and the static attitude measurements is a manufacturing datum plane. The manufacturing datum plane is a horizontal plane representing a suspension mounting surface of an actuator. During manufacturing, the manufacturing datum plane is placed generally parallel to and below the suspension assembly.
A static attitude measurement includes a pitch axis angle measurement and a roll axis angle measurement measured in relation to the datum plane. The pitch and the roll axes are parallel to the horizontal plane and are mutually perpendicular, intersecting at a point on the head bonding platform. The roll axis is usually aligned with the longitudinal axis of the suspension assembly.
Z-height is often measured using a laser triangulation probe, also known as a point range sensor. The point range sensor produces a thin beam of light which is directed at a known angle to a point to be measured on the surface of the HSA. The beam of light is reflected by the surface of the HSA and strikes a light sensor array. The position of the reflected beam on the sensor array can be correlated by triangulation to a Z-height measurement. Laser triangulation offers fast point readings (measurements in less than 500 milliseconds) and can offer very good distance accuracy.
Static attitude can be measured using autocollimation systems. Autocollimation systems are measuring instruments that generate a collimated light beam (a light beam having parallel rays of light) having a diameter several (i.e., thirty to three hundred) times larger than the diameter of the light beam generated by the point range sensor system. The collimated light beam is directed to and reflected off the surface of the part being measured. The reflected light beam strikes a linear array of light sensors. The sensors collect data on the reflected light beam which is fed into a computer to calculate the pitch and roll angles of the part. Autocollimation systems offer accurate and fast angle measurements.
Manufacturing tolerances of HSA elements are commonly measured using one of two methods. The first method comprises placing the part to be measured on a measurement fixture and assuming that the Z-height value is close to correct based on how the part is fixtured. Of course, this means that the exact value of the Z-height measurement is unknown. The static attitude is then measured using an autocollimation system.
The second method again has the part placed in a measurement fixture. A moving point range sensor is used to scan across the surface of the part in both the x and the y directions, making Z-height measurements at interval points. An alternative to moving the sensor is to move the part while the sensor remains in a fixed position. A computer is used both to calculate the Z-height measurements and to compute a theoretical plane based on the scanned points. The theoretical plane is used for yielding the static attitude measurements. The repeated movement and positioning of the point range sensor and/or the part causes measurement errors along all three axis. The accuracy of the static attitude measurements depends on the number of scanned points. This method is time-intensive, due to the number of readings and movements required.
In theory, a third method of measurement is possible. The method would involve first using an autocollimation system to measure static attitude and then to either move the part and/or the probes to measure Z-height separately using a point range sensor. However, this two-station method would be time-consuming, require repeated movement of the part or the systems, and introduce severe positional errors. Repeated movement could cause damage to the HSA or the instruments.
The current measurement methods and instruments introduce significant measurement error sources and are time and manipulation intensive. There is a need for an alternative system for measuring the static attitude and Z-height of the parts of a HSA.